The present invention relates to a system for handling compressed gas. The invention also relates to a distribution system for supplying compressed natural gas to natural gas vehicles.
Natural gas (with methane as a major constituent) is typically supplied to end use installations at a relatively low pressure. For example, natural gas supplied through a pipeline is typically maintained at a pressure within a range of from five pounds per square inch (psig) to three hundred psig. However, at a natural gas vehicle refueling station, it may be desirable to store natural gas in compressed form at a high pressure. In particular, it may be advantageous to store the gas in uninsulated storage vessels at a pressure within a range of from about thirty-six hundred to about five thousand psig.
However, natural gas from a pipeline generally has a high moisture content. Natural gas supplied in bulk may have a moisture content within a range of seven pounds of water per million cubic feet of gas (lbs/MMScf) to one hundred lbs/MMScf. When such wet gas is compressed from its low pipeline pressure to a high storage pressure, its dew point increases significantly.
For example, natural gas with a moisture content of seven lbs/MMScf, when pressurized to a storage pressure of thirty-six hundred psig, will have a dew point of about fifty-two degrees Fahrenheit. This is a problem because it is of course desirable to locate natural gas vehicle refueling stations in cold climates where uninsulated storage vessels would encounter temperatures far below fifty-two degrees Fahrenheit. When the temperature of a compressed gas drops below its pressure dew point (in this example, fifty-two degrees Fahrenheit), the moisture within the gas condenses out as liquid water. Condensed water will promote corrosion, especially in the presence of other impurities like carbon dioxide and hydrogen sulfide. Liquid water is also disadvantageous because it displaces gas and thereby reduces storage capacity. Also, if the temperature within the system were to fall below thirty-two degrees Fahrenheit, the water would freeze, disrupting or even blocking gas flow through the system.
To avoid condensation and freezing problems, natural gas handled at a refueling station should be dried to a pressure dew point that is less than the minimum temperature to which the gas is likely to be exposed. For example, if the lowest expected winter temperature for the refueling station is zero degrees Fahrenheit, then the gas should be dried to a pressure dew point within the range of from minus ten to minus twenty degrees Fahrenheit at thirty-six hundred psig. To achieve a pressure dew point of minus twenty degrees Fahrenheit at thirty-six hundred psig, the natural gas would have to be dried to about four-tenths of one pound of water per million cubic feet of gas.
Therefore, there is a need for an economical and effective system for dehydrating natural gas at vehicle refueling stations (and at other installations).
Systems for dehydrating natural gas have been developed. Of particular interest are dryers which contain desiccant material for removing water by adsorption. Prior art desiccant dryers for dehydrating natural gas are disclosed by U.S. Pat. Nos. 4,701,188 (Mims), 4,505,127 (Pronovost et al.), and 2,588,296 (Russel, Jr.). However, the prior art dryer systems have not been successfully employed at end user installations, and would not be satisfactory for use at natural gas vehicle refueling stations.